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    <title>Document</title>
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    <script>
        var reg = /^[0-9]$|^[1-9][0-9]$|^1[0-9][0-9]$|^2[0-4][0-9]$|^25[0-5]$/ ; 
        var a = prompt("请输入");
        // var str = '255' ;
        console.log(reg.test(a));
        
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